Method of drilling and completing a gas well

ABSTRACT

The well is drilled and cased to slightly above the gasproducing horizon. Drilling within the gas-producing horizon is performed by use of a nonaqueous drilling fluid. If the well is not sufficiently productive the adjacent formation is subjected to at least one stimulation by an explosive and test for adequate production. A hydraulic fracture network is induced by use of nonaqueous fracturing fluid, if appropriate stimulation is not achieved by the explosive means. The problem of irreversible wellbore damage created by water base drilling muds and aqueous fracturing fluids is avoided. Formation permeability restrictions in the vicinity of the wellbore are overcome by the use of the nonaqueous well completion technique.

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Inventors Vaughan W. Rhoades;

Eugene D. Glass, Tulsa, Okla.

Appl. No. 862,506

Filed Sept. 30, 1969 Patented Jan. 26, 1971 Assignee Cities Service OilCompany Tulso, Okla. a corporation of Delaware METHOD OF DRILLING ANDCOMPLETING A GAS WELL Primary Examiner-Ian A. Calvert Attorney-J.Richard Geaman ABSTRACT: The well is drilled and cased to slightly abovethe gas-producing horizon. Drilling within the gas-producing horizon isperformed by use of a nonaqueous drilling fluid. If the well is notsufficiently productive the adjacent formation is subjected to at leastone stimulation by an explosive and test for adequate production. Ahydraulic fracture network is induced by use of nonaqueous fracturingfluid, if appropriate stimulation is not achieved by the explosivemeans. The problem of irreversible wellbore damage created by water basedrilling muds and aqueous fracturing fluids is avoided. Formationpermeability restrictions in the vicinity of the wellbore are overcomeby the use of the nonaqueous well completion technique.

METHOD OF DRILLING AND COMPLETING A GAS WELL BACKGROUND OF THE INVENTIONThis invention relates to the recovery of gas from subterraneanreservoirs. More particularly it relates to the recovery of gasesfollowing the nonaqueous drilling of a gas formation and subsequentnonaqueous stimulation of said formation.

The use of nonaqueous drilling fluids in the completion of gas-bearingreservoirs is known in the art. The gas-bearing formation is drilled orcored by use of nonaqueous drilling fluid so that the cuttings or coresection and the gas-bearing formation obtained are not contaminated bywater and accurate laboratory tests may be performed to determine thetrue formation water content. It has been found that initial orsubsequent contacting of low permeability gas formation with aqueousdrilling fluids creates irreversible wellbore damage. This damagerestricts the production of the reservoir fluid and yields anexceptionally low well productivity. Stimulation of the damaged wellboreis normally not effective as there exists presently no satisfactorymeans of extracting the introduced water trapped in the formation duringdrilling.

As a result of this irreversible wellbore damage, many gas wells havebeen abandoned. Many of these reservoirs were extensively stimulated byaqueous fluid fracturing and thought not to be of productive capacity.With the use of a proper completion technique some of these wells wouldhave proved to be productive. Considerably higher well-drilling successratios would have resulted in enormous reserves of gas having beenreleased for industry consumption. A completion technique is requiredwhich can provide an unrestricted wellbore area from which theseformations may produce.

It is an object of this invention, therefore, to provide an improvedprocess for the recovery of gas from low permeability natural gasreservoirs.

It is another object of the invention to provide an improved techniquefor the completion of a subterranean gas reservoir.

It is another object of the invention to provide a technique forrecovering gas from a subterranean reservoir by the use of nonaqueousdrilling fluids and with proper stimulation obtained by the use ofnonaqueous fracturing means.

It is still a further object of the present invention to provide for theproper completion of subterranean gas reservoirs by the use ofnonaqueous drilling fluids and subsequent stimulation by nonaqueousfracturing means, and also to provide means for nonintroduction of waterbase drilling fluids, required for the completion of lower horizons,into the formations previously completed.

With these and other objects in mind, the present invention ishereinafter set forth.

SUMMARY OF THE INVENTION The objects of the invention are accomplishedby a technique in which drilling with conventional drilling fluids, forexample with water base muds, is ceased slightly above the gas-bearingformation. The wellbore is cased and cemented to the drilled horizonwith extreme care so as to seal the formation from the wellbore.Subsequent drilling through the low permeability gas-bearing formationis accomplished by use of a nonaqueous drilling fluid. Formationproductivity is determined by conventional well testing means. Ifstimulation is required a sequence of explosive charges is detonated inthe wellbore opposite the gas bearing horizon. If proper stimulation isnot achieved with the use of the explosive charges, the well is thenfractured by use of a nonaqueous fracturing fluid. Means are provided toprotect the gas-bearing formation wellbore should further drilling withwater base muds be required.

DETAILED DESCRIPTION OF THE INVENTION The drilling method used prior topenetration of the gasbearing formation may consist of any conventionaldrilling practice such as rotary drilling with a water base mud. .Extreme'care is required as the drill bit approaches the gas-bearingformation in order that no drilling mud filtrate is allowed to permeatethe gas-bearing strata. All drilling with aqueous drilling fluid shouldbe ceased far enough from the gas-bearing formation such that no aqueousfluid contacts or permeates the formation. Generally discontinued use ofaqueous drilling fluids, 5 to l0 feet above the formation, is adequate.Low water loss muds or anhydrous base muds should be used to helpalleviate the filtrate invention problem. These muds should have a highcolloidal content so as to form a filtrate cake of low permeabilitywhich will prohibit circulation fluid loss to the gas-bearing formation.It is preferred in the embodiment of our invention to use an aqueouscolloidal bentonitic clay-drilling mud while drilling in the vicinity ofthe gas-bearing formation whereby the rate of drilling fluid filtrateloss will be substantially reduced.

The casing and cementing operation should be performed with greatcaution to avoid any gas-bearing formation invasion by cement filtrate.The sealing agent used may be any oil well cement which is pumped downthe casing as a slurry and upward into the annulus where it is allowedto harden and set. Mud cake remaining on the walls of the hole from thewater base-drilling mud should be washed or eroded away so as to permitan appropriate bonding of the cement between the easing and walls of thehole. A washing procedure with water is customarily used to alleviatethe residual mud cake and allow for the proper cementing of the casingto the well wall. Often this operation is supplemented by applying smallscrapers attached to the casing, and reciprocating the pipe in the holeto clear any residue caking. However, in accordance with this invention,the mud cake is removed from the well wall by rinsing with a nonaqueousfluid. This procedure is to prevent any invasion of the gas-bearingformation by aqueous fluid which will create irreversible wellboredamage. The nonaqueous washing fluid also forms a barrier about thewellbore which restricts further invasion by the cement filtrate.Squeeze cementing may be required to complete the shutoff which may nothave been entirely affected by the normal casing cementing job. Squeezecementing such as with a low water loss cement is preferred to avoid thewater damage so defined.

The gas-bearing formation may be drilled by use of any of the commonlyused drill bits. While drilling through the gas bearing formation asuitable nonaqueous fluid should be used as a replacement for thedrilling mud. The preferred nonaqueous drilling fluid should be chosenfrom the group consisting of readily available dry gases such as air,nitrogen, natural gas, and carbon dioxide but low viscosity, nonaqueousliquids having high volatility such as liquified petroleum gas, lightnaphtha kerosene or other light end hydrocarbon distillation productsmay be used. Materials of sufiicient volatility are required as they areeasily withdrawn from the formation and,

therefore, do not create any irreversible wellbore damage.

The nonaqueous drilling of the gas-bearing formation should be ceasedabove any bottom water.

First the productivity of the gas reservoir is determined. Conventionalopen-flow testing and bottomhole pressure meas'urements are the mostcommon form of productivity test, but any applicable method may beemployed. If the reservoir is not sufficiently productive, a series ofexplosions in the wellbore followed by further productivity testing maybe required. Should productivity still be insufficient after theexplosive stimulation, the gas reservoir is subjected to fracturing witha nonaqueous fluid. Suitable fracturing fluids would include thosechosenfrom the group consisting of air, nitrogen, natural gas, carbondioxide, or any suitable normally gaseous material. Low viscositynonaqueous liquids having a high volatility such as liquified petroleumgas, light naptha kerosene and other light end hydrocarbon distillationproducts may also be employed as the fracturing fluid. One or morefracturings may be required with the use of propping agents such assand, nutshells, polymer beads, etc. in order to render thereservoirproductive.

The particular problem which is overcome by our invention is illustratedby the example. presented'herein. The following table represents theresults of the permeability reduction experienced in a low permeabilitygas formation due to irreversible wellbore damage. The experimentalresults were obtained by confining a two-inch diameter by two-inch longcore plug under different reservoir pressures, injecting nitrogen, asuitable nonaqueous drilling fluid, and measuring the permeability ofthe core to the nitrogen, the natural permeability of the rock matrix,as would exist if nonaqueous drilling fluids had been used during thewell drilling. Subsequently the core plug was subjected to waterinjection as would exist from the use of water base-drilling fluids andthe permeability to nitrogen was thereafter measured at the samereservoir pressures as for the natural permeability test. Therefore, theTable represents the percentage increase in wellbore permeabilityobtained by utilizing nonaqueous drilling fluid rather than a water basedrilling fluid.

TABLE Percentage increase in permeability obtained by utilizingnonaqueous drilling Wellbore Stress, p.s.i.: fluids The aboveexperimental test was conducted using a western Oklahoma sandstone coreplug. Similar results have been obtained for other formation types andfor a variety of permeability ranges. The results obtained in all testshave shown a marked increase in wellbore permeability and wellproductivity by the use of nonaqueous drilling fluids and show theutility of our present invention.

Another embodiment of our invention is the multiple drilling andcompletion of several gasbearing formations in a single production well.The first gas-bearing formation is substantially drilled and completedas discussed above.

If subsequent formations are to be drilled at depths considerably belowthat of the first gas bearing formation, a solid well liner should beset within the wellbore of the first gas bearing formation beforeconventional water base-drilling fluids are reintroduced into the hole.The method by which the liner is set is not a constraint upon thepresent invention. However, this procedure should be consistent with theconcept of the invention in that no appreciable aqueous substance shouldbe contacted with the gas-bearing formation. This liner will allow theuse of water base-drilling fluids for the drilling of the intermediatestructures and protect the first gas-bearing formation from mud filtratewellbore damage. Each additional gas bearing formation is drilled,stimulated, tested, and lined as described with the last formation leftunlined and open. The productivity of each formation is tested byconventional test ing methods. Those formations thought to be productivebut which do not so indicate upon original testing are stimulated byexplosives and retested. Multiple intervals of stimulation and testingmay be required in order to make the well produce sufficiently. Ifinsufficient stimulation is provided by the explosives the formationsshould be hydraulically fractured with a nonaqueous fracturing fluid.Preferred fluids are from the group consisting of readily available drygases under pressure such as air, nitrogen, natural gas and carbondioxide although low viscosity, nonaqueous liquids having a highvolatility such as liquified petroleum gas, light naphtha, kerosene andother light end hydrocarbon distillation products may be used. Multiplenonaqueous fracturing jobs may also be required to sufficientlystimulate the well. Standard propping agents may be incorporated withthe fracturing fluid to sustain the fracture system created in theformation.

When all formations of possible interest have been drilled any waterzones open to the well are shutoff to the production string. Theisolation of these water-containing strata may be effected by the use ofpackers or casing set with a cement liner. As in all cases extreme careshould be used so as not to allow any aqueous fluid introduction intothe gas-bearing formations. After all gas-bearing formations have beendrilled the zones found to be productive are perforated. All gas-bearingformations determined to be productive are connected to one or moreproduction lines and readily produced without production reduction dueto irreversible wellbore damage.

The present invention, therefore, provides a highly signifcant practicalmethod for the completion of wells in low permeability gas reservoirs.The irreversible wellbore damage created during conventional drillingsuch as with water base muds is eliminated by the use of the successfulcompletion method provided. By this method tight reservoirs, which werepreviously abandoned, may be rendered productive and economic. Recoveryof natural gas from what were previously unproductive subterranean gasreservoirs is thereby enhanced.

The invention has been described herein with respect to particularembodiments and aspects thereof. It will be appreciated by those skilledin the art that various changes and modifications can be made, however,without departing from the scope of the appended claims.

We claim:

1. A method for recovering natural gas from a subterranean gas reservoircomprising:

a. initially drilling a well using an aqueous drilling fluid to a depthslightly above the gas reservoir so that no aqueous drilling fluidpermeates the gas reservoir;

b. casing and cementing the well to said depth;

c. subsequently drilling the well through the gas reservoir with anonaqueous drilling fluid; and

d. stimulating the gas-bearing formation by nonaqueous means.

2. The method of claim 1 in which said nonaqueous drilling fluid isselected from the group consisting of dry gases and low viscositynonaqueous liquids having high volatility.

3. The method of claim 1 in which said stimulation of the gas-bearingformation by nonaqueous means consists of one or more series ofexplosive detonations within the wellbore.

4. The method of claim 3 in which said series of explosive detonationsis followed by one or more formation fracturings with a nonaqueousfracturing fluid selected from the group consisting of dry gases and lowviscosity nonaqueous liquids having high volatility.

5. The method of claim 1 in which said initial drilling method utilizesan aqueous colloidal bentonitic clay in the vicinity of the gas-bearingformation, thereby forming a mud cake on the well wall which restrictsthe permeation of the mud filtrate into the gas reservoir.

6. The method of claim 5 in which the mud cake formed by said bentoniticclay is removed from the well wall by rinsing with a nonaqueous liquidprior to said casing and cementing.

7. A method for recovering natural gas from several subterraneangas-bearing formations comprising:

a. initially drilling a well using an aqueous drilling fluid to a depthslightly above the first gas-bearing formation so that no aqueousdrilling fluid permeates the formation;

b. casing and cementing the well to said depth;

c. subsequently drilling the well through the gas-bearing formation witha nonaqueous drilling fluid;

d. stimulating the gas-bearing formation by nonaqueous means;

e. isolating the gas bearing formation from the well by a solid linerover the depth interval of the gas-bearing formation; and

f. sequentially and repetitiously drilling and isolating subsequentgasebearing formations as described in steps a) through e) leaving thelast gas formation to be drilled exposed to the well.

8. The method of claim 7 in which said nonaqueous drilling fluid isselected from the group consisting of dry gases and low viscositynonaqueous liquids having high volatility.

9. The method of claim 7 in which said stimulation of gas bearingformations by nonaqueous means consists of one or more series ofexplosive detonations within the wellbore.

10. The method of claim 9 in which said series of explosive detonationsis followed by one or more formation fracturings with a nonaqueousfracturing fluid selected from the group consisting of dry gases and lowviscosity liquids having high volatility.

11. The method of claim 7 in which said drilling in non gasbearingformations utilizes an aqueous colloidal bentonitic clay in the vicinityof the gas-bearing formations, thereby forming a mud cake on the wellwall which restricts the per meation of the mud filtrate into the gasreservoir.

12. The method of claim 11 in which the mud cake formed by saidbentonitic clay is removed from the well wall by rinsing with anonaqueous liquid prior to said casing and cementing.

13. The method of claim 7 further comprising perforating said liners ofthe productive gas-bearing formations so that gas production may becommenced.

2. The method of claim 1 in which said nonaqueous drilling fluid isselected from the group consisting of dry gases and low viscositynonaqueous liquids having high volatility.
 3. The method of claim 1 inwhich said stimulation of the gas-bearing formation by nonaqueous meansconsists of one or more series of explosive detonations within thewellbore.
 4. The method of claim 3 in which said series of explosivedetonations is followed by one or more formation fracturings with anonaqueous fracturing fluid selected from the group consisting of drygases and low viscosity nonaqueous liquids having high volatility. 5.The method of claim 1 in which said initial drilling method utilizes anaqueous colloidal bentonitic clay in the vicinity of the gas-bearingformation, thereby forming a mud cake on the well wall which restrictsthe permeation of the mud filtrate into the gas reservoir.
 6. The methodof claim 5 in which the mud cake formed by said bentonitic clay isremoved from the well wall by rinsing with a nonaqueous liquid prior tosaid casing and cementing.
 7. A method for recovering natural gas fromseveral subterranean gas-bearing formations comprising: a. initiallydrilling a well using an aqueous drilling fluid to a depth slightlyabove the first gas-bearing formation so that no aqueous drilling fluidpermeates the formation; b. casing and cementing the well to said depth;c. subsequently drilling the well through the gas-bearing formation witha nonaqueous drilling fluid; d. stimulating the gas-bearing formation bynonaqueous means; e. isolating the gas bearing formation from the wellby a solid liner over the depth interval of the gas-bearing formation;and f. sequentially and repetitiously drilling and isolating subsequentgas-bearing formations as described in steps a) through e) leaving thelast gas formation to be drilled exposed to the well.
 8. The method ofclaim 7 in which said nonaqueous drilling fluid is selected from thegroup consisting of dry gases and low viscosity nonaqueous liquidshaving high volatility.
 9. The method of claim 7 in which saidstimulation of gas-bearing formations by nonaqueous means consists ofone or more series of explosive detonations within the wellbore.
 10. Themethod of claim 9 in which said series of explosive detonations isfollowed by one or more formation fracturings with a nonaqueousfracturing fluid selected from the group consisting of dry gases and lowviscosity liquids having high volatility.
 11. The method of claim 7 inwhich said drilling in non gas-bearing formations utilizes an aqueouscolloidal bentonitic clay in the vicinity of the gas-bearing formations,thereby forming a mud cake on the well wall which restricts thepermeation of the mud filtrate into the gas reservoir.
 12. The method ofclaim 11 in which the mud cake formed by said bentonitic clay is removedfrom the well wall by rinsing with a nonaqueous liquid prior to saidcasing and cementing.
 13. The method of claim 7 further comprisingperforating said liners of the productive gas-bearing formations so thatgas production may be commenced.